The present invention relates to an improvement of a device for delivering as digital data the quantity of electricity and its functional values in the case of a failure of an electric-power system such as disconnections or ground faults during the failure time interval or after the recovery of the failure.
Since the practical introduction of digital computation type protective relays utilizing computers, the magnitude of the voltage, the amount of current and the values of impedance and its reactance measured by a distance relay have been obtained as digital data, but with such data it and is impossible to clarify the quantity of electricity between a plurality of electric stations.
When a fault is limited to one point in an electric-power system, is so simple that the fault mode remains unchanged and can be interrupted by the correct operation of a protective relay, there is no problem in general to analyze the fault and the response of the protective relay even though the phase relationship between a plurality of quantities of electricity is not clear. However, when a fault results in complicated modes such as
(i) when faults occur at a plurality of points simultaneously (multiple faults);
(ii) when disconnections and ground faults occur simultaneously (disconnection/ground fault); and
(iii) when the number of phases of faults varies during a fault time interval (evolving fault)
and when the voltage, the current phenomenon and the response of the protective relays are analyzed at the time when these faults occurred in which they are sequentially interrupted, it is very advantageous to clarify the mutual relationships of quantities of electricity among a plurality of electric stations during a fault time interval.
Furthermore, in the case of determining a fault point by using the voltage and the current in an electric-power system, when the mutal relationships of the quantities of electricity among a plurality of electric stations are clarified, errors can be minimized in most cases. These mutual relationships will be described below with reference to FIG. 8 which is a circuit diagram used to explain the phenomenon resulting from a fault of a two-terminal transmission line. That is, L is a transmission line; A and B are terminals thereof; and F is a fault point. The currents I.sub.A and I.sub.B flow from the terminals A and B, respectively, and the current I.sub.F flows at the fault point F which has a fault-point resistance R.sub.F. When the impedance of the transmission line is Z.sub.L per kilometer and the distance between the fault point F and the terminal AF is x km, then the impedance between A and F becomes xZ.sub.L. The voltage at the terminal A is V.sub.A and is given by EQU V.sub.A =I.sub.A .times.Z.sub.L +I.sub.F R.sub.B ( 1)
In this case, the most precise distance can be obtained from the following equation: EQU x=V.sub.A sin .PHI..sub.V /I.sub.A Z.sub.L sin .PHI..sub.I ( 2)
where
.PHI..sub.V is the angle of lead of V.sub.A relative to I.sub.F, and
.PHI..sub.I is the angle of lead of I.sub.A Z.sub.L relative to I.sub.F.
FIG. 9 is a vector diagram used to explain the above-described relationship and shows some phase difference between the currents I.sub.F and I.sub.A.
The voltage V.sub.A is expressed by two terms in Eq. (1), R.sub.F is a pure resistance and Z.sub.L is an impedance having a high inductance component so that I.sub.F R.sub.F are in phase with I.sub.F and I.sub.A Z.sub.L leads in phase slightly by 90.degree. from I.sub.A. The numerator V.sub.A sin .PHI..sub.V and the denominator I.sub.A sin .PHI..sub.I are represented by the projections, respectively, on the line o-l of V.sub.A and I.sub.A Z.sub.L. Since the projection of V.sub.A is equal to the projection xI.sub.A Z.sub.L, the precise distance x is obtained from Eq. (2).
One of the features of the above-described principle resides in the fact that the influence of the voltage drop I.sub.F R.sub.F across the fault-point resistor is eliminated so that the detection of a fault point distance can be measured with a high degree of accuracy, but in order to measure the current I.sub.F, it is required to obtain the amount of the current I.sub.B and the relative phase angle between the current I.sub.B and the current I.sub.A.
FIG. 10 is a view used to explain the measurement of a fault-point distance on a transmission line with three terminals. The same reference symbols are used to designate similar parts both in FIGS. 8 and 10. In FIG. 10, C is a third terminal of the transmission line L and J designates the junction point. The current I.sub.C flows from the terminal C. The impedance between the terminal A and the junction J is Z.sub.AJ ; the distance between the junction J and the fault point F is .times.km; and the impedance per km is Z.sub.L.
In this case, even if the desire for eliminating the influence of an error due to the voltage drop I.sub.F R.sub.F across the fault-point resistance obtained by the fault-point current I.sub.F is given up, a further problem arises. That is, when the fault point F exists between the terminal C and the junction J and if the power source connected to the terminal C is weak or low, the current I.sub.C hardly flows so that the distance measurement at the terminal C becomes impossible. It follows therefore that the distance measurement must be made at the terminal, for instance, A which is connected to a high backup power supply and at which a high amount of fault current flows.
In this case, the voltage V.sub.A -I.sub.A Z.sub.AJ is given by the following equation: EQU V.sub.A -I.sub.A Z.sub.AJ =(I.sub.A +I.sub.B).times.Z.sub.L +I.sub.F R.sub.F ( 3)
and as in the case of Eq. (2), the distance x is given by the following equation: EQU x=.vertline.V.sub.A -I.sub.A Z.sub.AJ .vertline.sin .PHI..sub.V '/.vertline.(I.sub.A +I.sub.B)Z.sub.L .vertline.sin .PHI..sub.I '(4)
where
.PHI..sub.V ' is the angle of lead of V.sub.A -I.sub.A Z.sub.AJ relative to I.sub.F, and
.PHI..sub.I ' is the angle of lead of (I.sub.A +I.sub.B)Z.sub.L in relation to I.sub.F.
In this case, even when the current I.sub.F is in phase with I.sub.A +I.sub.B or I.sub.A and even if the distance to the fault point is measured without the use of the fault-point current I.sub.F, the current I.sub.A +I.sub.B must be calculated and the relative phase angle between the currents I.sub.A and I.sub.B must be obtained.
Furthermore, after the recovery of fault, the electric power and the differences in phase angle between the voltages at various points fluctuate and out-of-step is occurred in the worst case. It is very important to analyze precisely these phenomena in order to ensure the stable operation of an electric-power system. To this end, so far the electronic computers have been used, but in the actual systems, the recorded data are only the electric power, the voltage and the amount of current. The difference between the results obtained by analysis and the results actually obtained has not been satisfactorily clarified and in order to clarify this difference, it becomes necessary to clarify the relative phase angles among the voltages at various stations after the recovery of fault.
As described above, various advantages can be attained by obtaining the data which can clarify the relative phase angles among the voltages at various stations during a fault time interval or after the recovery of fault. However, so far no means has been available for obtaining such data as described above. It is of course possible to detect the relative phase angles by transmitting the waveform of an instantaneous voltage or current value to a remote point by a PCM process or a frequency modulation process, but this method has a defect that a large amount of information must be transmitted at a high speed.